This invention relates to a system for electronically modulating a musical tone signal, and, more particularly, to a system for modulating a musical tone signal to simulate the radiation effects produced by a rotary loudspeaker.
The addition of pulsato, tremolo, chorus or other low frequency modulation effects to a musical tone signal enhances the richness of the resultant sounds. Pulsato may be produced using rotary sound channels, as shown in Leslie U.S. Pat. Nos. Re. 23,323, 3,080,786 and 3,174,579, among others. In one of the several embodiments disclosed in U.S. Pat. No. Re. 23,323, a high frequency speaker in the form of a directional horn and a lower frequency speaker are rotatably supported in a cabinet and are arranged to be rotated by respective motors. As the horn and low frequency speaker are rotated, not necessarily synchronously, the pitch of the sound reaching the listener's ear varies and, by appropriately choosing the speed of rotation, a pleasing pulsato effect is obtained. The patent teaches that best results are frequently obtained by rotating the speakers at different speeds and in opposite directions, implying that the relative phase of the signals from the two speakers continously varies. While possibly not recognized by the inventor at the time, it was subsequently observed that a somewhat different tremolo is produced at higher frequencies than at the lower end of the spectrum, and that the high frequency rotating horn produces an effect comparable to that occurring in a pine organ at the transition near the top end of the rank from wooden to metal pipes; the small metal pipes react much differently to variations in air pressure and produce a different and much deeper vibrato and tremulant effect than do the wooden pipes. While systems of the general configuration taught by Leslie have enjoyed wide and long-term acceptance, many investigators have attempted to electronically simulate the desirable effect in order to eliminate the bulk and cost of the rotary speakers, and the attendant mechanical problems.
One such electronic system is known from U.S. Pat. No. 4,008,641 which has three channels each coupled to a respective loudspeaker and each having an amplitude modulator therein. A tone signal to be modulated is applied directly to the amplitude modulator in one of the channels and through a delay circuit to the amplitude modulator in each of the other two channels. A frequency modulator is coupled to the amplitude modulator in the first channel and to the delay circuit for frequency modulating the musical tone signals therein, and phase shifters are coupled between the frequency modulator and the respective amplitude modulators in the second and third channels for shifting the phase of the musical tone signal in these channels. The outputs of the amplitude modulators are acoustically reproduced, with the tone signal from the first channel being in the center of the reproduced sound image and the musical tone signals from the other channels on opposite sides of the tone signal from the first channel. The sound emanating from the center speaker is loudest at the transition between sharp and flat of the frequency modulated signal, and one of the side speakers is loudest when the frequency modulated signal is going from flat to sharp while the other side speaker is loudest when the FM signal is going from sharp to flat; this produces the effect of rotation, but does not accurately simulate the acoustic effects produced by a rotary speaker. That is, when the FM modulated signal is going sharp, the signal produced by one of the side speakers is more dominant than it should be. Moreover, proper operation of the system is highly dependent on the relative placement of the speakers, and also requires rather specific positioning of the listener with respect to the speakers for him to perceive a rotating sound effect.
A device for electronically simulating the radiation effects produced by a rotary speaker, which requires only two loudspeakers, is described in U.S. Pat. No. 4,162,372. In this system, an input tone signal is frequency modulated at a sub-audio rate and the frequency modulated signal and the original signal are mixed and applied to two variable gain amplifiers, the outputs of which are applied to respective loudspeakers. The gains of the amplifiers are varied in phase opposition at the aforementioned sub-audio frequency, the modulating signal being applied to the amplifiers through a low-pass filter having a crossover at about 1.0 Hz, so that the amplitude modulation is more pronounced at 0.7 Hz than at 7 Hz. This quite closely simulates the effect in a rotary speaker pulsato generator wherein amplitude modulation is less distinct in the "fast" mode than in the "slow" mode, but because the amplitude modulation occurs in both channels in synchronism, the system does not simulate the effect of a rotary speaker facing away from the listener.
Thus, these two known systems, while each simulates to a degree many of the characteristics of the sound produced when a rotary speaker is used to modulate a musical tone signal, fail to simulate other effects, with the consequence that neither accurately simulates the pulsato and radiation effects produced by a rotary loudspeaker. Moreover, the system of U.S. Pat. No. 4,008,641 is relatively expensive to manufacture and, as has been previously noted, requires a particular placement of the loudspeakers relative to each other, and rather specific positioning of the listener with respect to the speakers, to realize the desired results.
Another device for electronically modulating a musical tone signal to produce substantially the radiation effects produced by a rotary loudspeaker is described in commonly assigned application Ser. No. 107,220 filed concurrently herewith by George F. Schmoll III. This system employs two loudspeakers and has an amplitude modulator associated with one and a variable delay device associated with the other. The musical tone signal is applied to both the amplitude modulator and the variable delay device, and both the variable delay device and the amplitude modulator are modulated synchronously by a sub-audio frequency modulating signal. The amplitude modulator delivers an output to its loudspeaker only during positive half-cycles of the modulating signal, during which time the frequency modulated signal produced by the variable delay device is going from flat to sharp, and during the period that no amplitude modulated tone signal is produced the frequency modulated signal is going from sharp to flat, whereby when the separately reproduced modulated signals are acoustically mixed a rotating sound effect is produced.
It is an object of the present invention to provide a circuit for electronically modulating a musical tone signal to generate tremulant effects simulative of those produced by a rotary loudspeaker. Another object is to produce radiation effects simulative of those produced by a rotatable tremulant sound producer of the type having both low and high frequency rotating channels.